1. Field of the Invention
This invention relates to a method of reaction injection molding of thermosetting polymeric material, whereby procedures are utilized to optimize parameters relating to the molding process.
2. Description of the Prior Art
Reaction injection molding (RIM) is polymerization and processing in a single cyclic operation. It consists of several distinct processes. Reservoirs of one, two, or more components are metered according to a predetermined ratio and then conveyed under pressure to a mixing head. The fluids are either mixing activated in the head or thermally activated upon injection into the mold cavity. Typical polymer resins utilized in RIM include polyurethane, epoxy, polyester, and nylon, with other thermoset materials being possible.
Injection into the mold is usually at modest mass flow rates to insure smooth laminar filling and avoid instabilities commonly known as fingering. Considerable viscosity increase can occur during cavity filling; the extent depending on the fill time and resin reactivity. Most of the chemical conversion usually occurs during the mold curing step after the mold has been filled. The part is ejected after some fixed mold residence time which allows the part to attain dimensional stability. A thermoset part may be ejected while still hot because it is crosslinked, but a post cure treatment may also be required. The cycle time includes the time for cavity filling, mold curing, and mold preparation, and it is typically desirable for it to be a minimum. Excessive cycle times due to long cure times may eliminate or limit the use of a molding resin in a particular application.
Material and process parameters determine the moldability of a resin in a particular application. Characteristic properties of a resin such as the gel time and the time to attain an ejectable rigidity are strongly influenced by temperature, both the temperature of the components and the mold temperature. Higher temperatures provide for more rapid reactions and shorter reaction times, but they can also increase the viscosity rise during cavity filling and the maximum exotherm temperature. Too high a resin viscosity during cavity filling can lead to plugging the cavity before it is completely filled, referred to as a "short shot". The necessity to avoid this effect can limit the design complexity of the cavity to only simple shapes that can be rapidly filled. Thermal runaway is often encountered in reactive polymer processing, and it is often desirable to minimize the cycle time. High exotherm temperatures are particularly important when they can result in polymer degradation or damage to an encapsulated electronic device.
In the past, to reconcile the various material and processing parameters typically required constructing a mold and attempting to fill it with the desired resin. If any of the various problems noted above (among others) occurred, then the mold had to be redesigned or the resin changed. Mold design is typically a costly custom process. Furthermore, with little assurance that a new design will work, mold shapes and processing cycles are necessarily more conservative than would be the case if more accurate mold filling information were available.